Among the available thin-film technologies, silicon thin-film technology has potential as the technology allows for the manufacturing of tandem and triple junction cells which can provide the high efficiency needed in today's solar cell market.
Amorphous silicon (a-Si:H) has been combined with silicon wafers for heterojunction tandem solar cells with success. For example, Panasonic sells solar panels called “HIT,” which stands for Heterojunction with Intrinsic Thin layer. This silicon heterojunction technology involves hydrogenated a-Si (or a-Si:H) combined with silicon wafers and is being explored by various companies and research groups. Such solar cells are also referred to as “Si-HJT” or “silicon heterojunction technology” cells. Sanyo has made solar cells from this technology with 23% efficiency.
The a-Si:H/c-Si heterojunction technology (“HJT”), combines the advantages of crystalline silicon wafer solar cells with the excellent absorption and passivation characteristics of amorphous silicon.
To produce the electric structures of heterojunction cells, it is necessary to apply thin layers of doped and intrinsic amorphous silicon on both sides of n-type silicon wafers as well as transparent conductive oxide layers (TCO) to absorb the generated power.
As a result of the high light yield and outstanding passivation characteristics of amorphous silicon, it is possible to reach efficiency rates of more than 22%. Moreover, a-Si:H/c-Si heterojunction cells show a considerably lower temperature coefficient than conventional silicon wafer (c-Si) solar cells.
Heterojunction, a-Si:H/c-Si technology, can only be realized on n-type monocrystalline material and the maximum efficiency rate lies currently at about 21%.
Heterojunction solar cells have also been made combining a-Si:H with microcrystalline silicon (μc-Si:H) thin films with success. This is a second type of a-Si tandem cell technology on the market today. While this is called microcrystalline (or sometimes nanocrystalline) silicon, it is actually still—perhaps misleadingly—amorphous (with very small crystals) and should not be confused with crystalline thin-film silicon (CSiTF)—which is polycrystalline or textured. For example, the best module efficiency in a microcrystalline silicon solar cell is achieved when the Raman crystallinity of the intrinsic microcrystalline layer is between 45% and 60% or nearby the so called “transition phase” with an a-Si:H and μc-Si:H mixed phase material (J. E. Hoetzel et al “Microcrystalline bottom cells in large area thin film silicon MICROMORPH™ solar modules”, May, 2016). The Raman crystallinity of CSiTF on the other hand should be as close to 100% (greater than 90%) as possible and ideally of pure, single phase (not in transition).
A-Si:H/μc-Si:H thin-film silicon tandem cells have been investigated by many groups. One company, Tel Solar AG, termed their cell “Micromorph™” which is a combination of the words MICROcrystalline and aMORPHous. So far the world record for a-Si:H/μc-Si:H or Micromorph™ cells is 12.34% (J. E. Hoetzel et al “Microcrystalline bottom cells in large area thin film silicon MICROMORPH™ solar modules”, May, 2016). A-Si/μc-Si thin-film silicon “hybrid” tandem cells are currently for sale by Kaneka Solar Corporation (their efficiencies are around 10%).
As stated above, both a-Si:H/c-Si and a-Si:H/μc-Si:H (or Micromorph) technologies are actually used in the current market place and panels made from these technologies can be purchased. However, despite the success of these technologies, efficiencies are still limited and need to surpass 25% efficiency to become truly competitive with the current state of the art crystalline silicon panels and those which are rapidly emerging. What is needed therefore is an a-Si tandem architecture that enables much higher efficiency. The present invention meets this need by replacing the silicon wafer (c-Si) or microcrystalline silicon (μc-Si) under layers with CSiTF.